This study is designed to address and resolve as-yet unanswered questions about the effects and effectiveness of a lower-than- standard range of lithium (O.4-0.6 mEq/1) in the maintenance treatment of patients with bipolar effective illness. We are comparing and contrasting this lower level with a more standard one (08-1.0 mEq/1) regarding protection against relapse, the presence of less severe effective symptoms, and association with unwanted and toxic effects. This double-blind study is being run concomitantly at both the Hillside Hospital, Division of Long Island Jewish Medical Center and the Massachusetts General Hospital. Projected intake to completion is 175 subjects at the two sites, with patients randomly assigned to the two different treatment regimens.